Sunbury Exhausts did my twin system..
Was a custom job, though they were able to supply redback extractors (for my I6), all mounting hardware, twin redback mufflers and chrome tips. 2.25" mild steel, cost me just under $1500.
I am very happy with the job they did, it sounds great, the underside is very neat, welds and everything look spot on and they got it done quickly.
http://www.sunburyexhaust.com.au/Haven't used them, but I know plenty of others who have and were happy with the job Sharp Exhausts did, they are also in Sunbury..
(For some reason their webpage won't load, but they have a facebook page too)
https://www.facebook.com/SharpExhausts